Jinsheng Yang

Extension of the modal wave-front reconstruction algorithm to non-uniform illumination.

Attempts are made to eliminate the effects of non-uniform illumination on the precision of wave-front measurement. To achieve this, the relationship between the wave-front slope at a single sub-aperture and the distributions of the phase and light intensity of the wave-front were first analyzed to obtain the relevant theoretical formulae. Then, based on the principle of modal wave-front reconstruction, the influence of the light intensity distribution on the wave-front slope is introduced into the calculation of the reconstruction matrix. Experiments were conducted to prove that the corrected modal wave-front reconstruction algorithm improved the accuracy of wave-front reconstruction. Moreover, the correction is conducive to high-precision wave-front measurement using a Hartmann wave-front sensor in the presence of non-uniform illumination.

Improving Retinal Image Quality Using Registration with an SIFT Algorithm in Quasi-Confocal Line Scanning Ophthalmoscope

When high-magnification images are taken with a quasi-confocal line scanning ophthalmoscope (LSO), the quality of images always suffers from Gaussian noise, and the signal to noise ratio (SNR) is very low for a safer laser illumination. In addition, motions of the retina severely affect the stabilization of the real-time video resulting in significant distortions or warped images. We describe a scale-invariant feature transform (SIFT) algorithm to automatically abstract corner points with subpixel resolution and match these points in sequential images using an affine transformation. Once n images are aligned and averaged, the noise level drops by a factor of [Formula: see text] and the image quality is improved. The improvement of image quality is independent of the acquisition method as long as the image is not warped, particularly severely during confocal scanning. Consequently, even better results can be expected by implementing this image processing technique on higher resolution images.

Optical Design of Adaptive Optics Confocal Scanning Laser Ophthalmoscope with Two Deformable Mirrors

We describe the optical design of a confocal scanning laser ophthalmoscope with two deformable mirrors. Spherical mirrors are used for pupil relay. Defocus aberration of the human eye is corrected by a Badal focusing structure and astigmatism aberration is corrected by a deformable mirror. The main optical system achieves a diffraction-limited performance through the entire scanning field (6 mm pupil, 3 degrees on pupil plane). The performance of the optical system, with correction of defocus and astigmatism, is also evaluated.

Adaptive optical confocal fluorescence microscope with stochastic parallel gradient descent algorithm

We have demonstrated adaptive optics confocal microscopy with stochastic parallel gradient descent (SPGD) algorithm for in vivo vessel imaging of mice auricle. After aberration correction, better image quality and more clearly vascular structure were realized.

Design of a Compact, Bimorph Deformable Mirror-Based Adaptive Optics Scanning Laser Ophthalmoscope

We have designed, constructed and tested an adaptive optics scanning laser ophthalmoscope (AOSLO) using a bimorph mirror. The simulated AOSLO system achieves diffraction-limited criterion through all the raster scanning fields (6.4 mm pupil, 3° × 3° on pupil). The bimorph mirror-based AOSLO corrected ocular aberrations in model eyes to less than 0.1 μm RMS wavefront error with a closed-loop bandwidth of a few Hz. Facilitated with a bimorph mirror at a stroke of ±15 μm with 35 elements and an aperture of 20 mm, the new AOSLO system has a size only half that of the first-generation AOSLO system. The significant increase in stroke allows for large ocular aberrations such as defocus in the range of ±600° and astigmatism in the range of ±200°, thereby fully exploiting the AO correcting capabilities for diseased human eyes in the future.

Analysis of pinhole vector diffraction in visible-light

Using Hartmann-Shack (H-S) wave-front sensor to test lenses with high numerical aperture, the reference spherical wave-front from pinhole is used to calibrate the Hartmann sensor to improve the precision of calibration, but intensity uniformity of the reference spherical wave-front affects the precision of Hartmann sensor’s calibration. Based on the vector diffraction theory, intensity uniformity is calculated with finite-difference time-domain method in case of a converging Gaussian incident visible light on pinhole. In order to proof the correctness of the intensity model of pinhole vector diffraction, experimentation of intensity is performed in visible-light. When the pinhole is the material aluminum with thickness 200nm and pinhole diameter 500nm, the absolute error of intensity uniformity is about 2.57% and 2.31% within 0.75 NA and 0.5 NA of diffracted wave-front by comparing experiment result with simulation result, so the intensity model is accurate.

Measurement of corneal topography through Hartmann-Shack wave-front sensor

A corneal topography based on Hartmann-Shack Sensor is presented in this paper. In the system, the focus of an objective lens is precisely positioned on corneas curve center. Wave-front of the reflecting beam can be measured by the Hartmann-Shack sensor which is conjugate to the cornea plane. If the corneal surface is a perfect sphere, wave-front detected by the Hartmann-Shack sensor is a plane. As a result, data measured by Hartmann-Shacks sensor is the deviation between the sphere and the real cornea surface. This paper describes a methodology for designing instrument based on Hartmann-Shack sensor. Then, applying this method, an instrument is developed for accurate measurement of corneal topography. In addition, measuring principle of Hartmann-Shack sensor which determined system parameters is also introduced. Repeatability is demonstrated by a series of data. The instrument was able to accurately measure simulative corneas reflective aberrations, from which corneal topography and corneal refractive aberrations were derived.

Extrapolation Method to Extend the Dynamic Range of the Shack-Hartmann Wave-front Sensor

The maximum tolerable spot displacement of the Shack-Hartmann wave-front sensor is called dynamic range, and it is limited by the f-number of the micro-lens. It is well known that there is a trade-off between the sensitivity and the dynamic range, if the f-number is relatively short, a high dynamic range is achieved, but the corresponding sensitivity is degraded and vice versa. Extrapolation method could overcome the trade-off; since the extrapolation method is a simple algorithm that does not change the measurement configuration, there is no requirement for extra hardware, multiple measurements, or complicated algorithms. In this paper, we not only present the theory but also actually implement measurement of a highly aerated wave-front.